The invention relates generally to a tool for forming bores through relatively hard material, and in particular to a rotary drill bit for use in oil and gas exploration and mining.
Bits for drilling through rock are typically outfitted with hard, durable cutters. Cutters with contact surfaces made from polycrystalline diamond compact (PDC) typically wear better and last longer. PDC is an extremely hard and wear resistant material.
PDC cutters are known to have one of the lowest rates of wear when operated at cooler temperatures. Wear rates are low when operational temperatures are maintained below about 700 degrees Celsius. At approximately 700 degrees Celsius, thermal damage to the diamond layer of the cutter begins, lowering wear resistance. Above this critical temperature, the rate of wear of the cutter can be as much as fifty times greater than the rate of wear at cooler temperatures. Consequently, PDC cutters become more susceptible to abrasive wear and breakage from impact when operating at higher temperatures.
Greater tangential cutter velocity causes more friction, thus generating more heat. Cutters moving at higher tangential velocities will thus tend to operate at higher temperatures. At some velocity, frictional heat reaches a level sufficient to cause cutter wear rates to accelerate, reducing the life of the cutters. In conventional PDC drag bits, the tangential velocity of a cutter, when measured relative to the material being cut, depends on the distance of the cutter from the center of rotation of the drill bit. For each rate of rotation of a drill bit of a particular diameter, further displacement of a cutter from the drill bit's axis of rotation proportionately increases the cutter's tangential velocity. Thus, increasing the diameter of a drill bit causes cutters located toward the periphery of the bit to rotate with greater tangential velocity.
Increased application of force also generates more heat. Cutters require more force to penetrate harder rock. Cutters dragging through harder rock have higher wear rates due to the increased application of force. Therefore, the critical point at which the wear rate begins to accelerate is also a function of hardness of rock in addition to the rotational velocity of the drill bit to which the cutter is attached. In softer rocks, accelerated wear rates do not occur until higher rotational speeds are used; in harder rocks, acceleration of the wear rate occurs at much lower rotational speeds.
A number of additional factors also shorten the life of PDC cutters. A cutter's abrupt contact with rock formations also increases the rate of wear of PDC cutters. Drilling with conventional PDC drag bits request application of weight and torque to a drill string to turn the drilling tool face and drive the face into the formation. Torque rotates the bit, dragging its PDC cutters through the formation being cut by the cutters. Dragging generates chips, which are removed by drilling fluids, thereby forming a bore or drilled hole. The drilling action causes a reverse, corresponding torque in the drill string. Because of the length of the drill string, the torque winds the drill string like a torsion spring. If a bit releases from consistent contact with the formation being drilled, the drill string will unwind and rotate backward. Changing the tension in the drill string causes the drill bit to come into irregular, abrupt contact either with the sides of the bore or the exposed formation surface being cut. These irregular contacts can cause impact damage to the cutters. Drill strings will vibrate, sometimes severely, under typical drilling conditions, a drill string rotates at 90 to 150 rpm. These vibrations can also damage a drill bit, including the cutters, as well as the drill pipe, MWD equipment, and other components in the drilling system. “Bit whirl” also contributes to impact loads on PDC cutters. This complex motion of the drill bit is thought to occur due to a combination of causes, including lateral forces on the drill bit due to vibration of the drill string vibration, heterogeneous rock formations, bit design, and other factors in combination with the radial cutting ability of PDC bits. Whirl of a drill bit in a bore subjects PDC cutters on the bit to large impact loads as the bit bounces against rock or other material in the bore. Cutters on these drill bits will lose large chips of PDC from impact, rather than from gradual abrasion of the cutter, thereby shortening the effective life of the cutters and the drill bit.
A drilling tool disclosed in U.S. Pat. No. 6,488,103 of Dennis et al., which is incorporated herein by reference, addresses one or more the problem of adverse thermal and impact effects on cutters and attempts to extend the life of PDC cutters without affecting drilling performance. The tool employs a plurality of satellite mills surrounding a central pilot bit. This arrangement reduces the tangential velocity at which cutters towards the periphery on the drilling tool face collide with material being cut by the drilling tool. A mud or turbine motor rotates the pilot and supplies power to drive shafts on which the satellite mills are mounted through a transmission. In order to avoid having to use seals to protect the bearings and gears from the down hole environment and to retain cooling lubricant, abrasion-resistant bearings and gear surfaces having PDC contact surfaces are used in the transmission. The drilling fluid lubricates and cools the transmission. The PDC surfaces enable the gears and the bearings to withstand the abrasion of the drilling fluids, cuttings and other debris present at the bottom of the hole.